This invention relates to compositions of matter and methods of using them to treat various industrial process streams, in particular certain compositions that have been found to be particularly effective in promoting the formation of sodium aluminosilicate (also known as desilication product or DSP) in a Bayer process stream.
As described among other places in U.S. Pat. No. 6,814,873, the Bayer process is used to manufacture alumina from raw bauxite ore. Because the Bayer process uses caustic solution to extract alumina values from bauxite it is cost prohibitive to perpetually use fresh caustic solution. As a result, a caustic solution known as “liquor” and/or “spent liquor” is recycled back from later stages of the Bayer process to earlier stages and thus forms a fluid circuit. For the purposes of this application, this description defines the term “liquor”. The recycling of liquor within the fluid circuit however has its own complexities.
Raw bauxite contains silica in various forms and amounts. Some of the silica is unreactive so it does not dissolve and remains as solid sand or mud within the Bayer circuit. Other silica (for example clays and kaolinite) is reactive and dissolves in caustic when added into Bayer process liquors. As spent liquor flows repeatedly through the liquor circuit of the Bayer process, the concentration of silica in the liquor increases eventually to a point where it precipitates, typically in conjunction with aluminum and soda to form insoluble sodium aluminosilicate. This material can precipitate as particulates in the liquor but is more often found as a hard scale on vessel walls in various parts of the Bayer process circuit. Aluminosilicate scale comes in at least two forms, sodalite and cancrinite. These and other forms of aluminosilicate scale are commonly referred to, and for purposes of this application define, the terms “desilication product” or “DSP”.
DSP has been variously described, in some cases it is considered to have a formula of 3(Na2O.Al2O3.2SiO2.0-2 H2O).2NaX where X represents OH—, Cl—, CO32—, SO42—. Because DSP has an inverse solubility (precipitation increases at higher temperatures) and can precipitate as scales of hard insoluble crystalline solids, the accumulation of silica in Bayer process liquor is problematic. Increased concentration of silica in solution leads to an increased propensity for precipitation of DSP. As solid DSP scale accumulates in Bayer process pipes, vessels, heat transfer equipment, and other process equipment, it forms flow bottlenecks and obstructions and can adversely affect liquor throughput. Because of its thermal conductivity properties, DSP scales on heat exchange also reduce the heat exchange efficiency. In addition, poor control of silica in solution can also affect the quality of the final alumina trihydrate product resulting in SiO2 contaminated alumina.
These adverse effects require plants to operate a range of control measures to mitigate the impact of silica dissolution on the functioning of the process. In terms of scale formation, one of the key issues is the significant downtime of Bayer process equipment that is required. Equipment is typically taken off-line as part of a routine descaling operation to remove scale buildup. In addition, DSP is difficult to remove and de-scaling requires the use of hazardous concentrated acids such as sulfuric acid.
Additionally, plants typically also incorporate a “desilication” step in the Bayer process. This step provides a controlled exit of silicate from the circuit (in the form of “free” DSP solids) and thereby mitigates the buildup of silica in solution. The desilication step is normally conducted prior to the digestion stage and removes some of the silica from the Bayer process liquids. Typically desilication is a process that involves maintaining Bayer slurry under conditions of temperature and holding time that are conducive to the precipitation of silica from solution in the form of sodalite (DSP) particles. Solid particles of sodalite that are formed under such conditions can then be removed from the process along with other insolubles (sand, mud) in the existing solid-liquid separation processes downstream. Conditions are typically arranged to minimize both the formation and impact of any DSP that may form as scale.
Some examples of desilication steps are described in international published applications WO 1996/006043, and WO 2006/003470, and published articles Product Silica Control Options, by B. J. Robson, Page 87, Light Metals, (1998), and A Novel Approach to Post-Desilicating Bayer Process Liquor, by K. I. The, Page 117, Light Metals, (1998). The efficient operation and removal of silica in the desilication process is a key process that plant operators use to control silica in solution. In this way, operators are able to mitigate the adverse impacts of high silica concentration, including product contamination and DSP scale formation. Such desilication steps however, are expensive and are not effective in removing all silica from solution. As a result, substantial quantities of dissolved silica typically pass on to subsequent Bayer process steps and so potential for DSP scale formation and product contamination remains. As a result, several other strategies have been suggested to control DSP scale in the Bayer process.
Another strategy is to reduce DSP scale in the Bayer process through the use of a DSP inhibitor. DSP inhibitors prevent the formation of DSP scale on Bayer process equipment, by inhibiting DSP precipitation and/or altering DSP morphology so it does not adhere to the equipment. Some examples of inhibitors are described in U.S. patent application Ser. No. 12/236946, U.S. Pat. No. 6,814,873 B2, US published applications 2004/0162406 A1, 2004/0011744 A1, 2005/0010008 A2, international published applications WO 2008/045677, WO 1997/041075, and WO 1997/041065, and published articles Max HT™ Sodalite Scale Inhibitor: Plant Experience and Impact on the Process, by Donald Spitzer et. al., Pages 57-62, Light Metals 2008, (2008) and Performance Appraisal of Evaporation System with Scale Inhibitor Application in Alnorte Plant, by A. Oliveira et al., Pages 133-136, Light Metals 2008, (2008). All of these attempts however involve tolerating the presence of silica in the Bayer process fluid circuit and compensating for the effects of the silica.
Another alternative strategy for addressing DSP scale is to enhance the removal of the silica upstream in the Bayer process. Increasing the mass of silica removed can result in a reduction of the concentration of silica in solution in subsequent, downstream processes. Such a result is likely to have some impact on DSP scale formation, and may also impact on product quality issues such as silica in alumina product.
Thus there is clear need and utility for an improved method of enhancing the removal of dissolved silica from Bayer process liquor in a controlled manner. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.